if I had a shop space available like what you say you have all of my jeep buddies would be invited any time all the time, along with chapter members - I just would not let strangers in...
I understand since he's charging the insurance liability is a consideration. How would it be if the space/tools were offered for free? Also if someone had a non-commercial shop at their home and offered it for free would home owners insurance cover accidents?
I expect if it were in someone's home and no money were involved then the risk would probably compare to a swimming pool. There are plenty of injuries and deaths that happen in swimming pools. This site claims 9 per day: https://www.edgarsnyder.com/swimming-pool/swimming-pool-statistics.html
That certainly doesn't keep people from having pools or letting folks come in and use them. I expect insurance deals with it in some fashion.
Apply the situation to a commercial environment and start asking folks to pay a price (probably won't matter that the price is only enough to cover overhead, you are already an evil capitalist pig) and I doubt insurance would be inclined to step up to the plate unless you were already upfront with them about it and paying the requested premium for that level of risk.
It is going to come down to a personal choice about a level of risk tolerance. Just about everything involves some level risk. Each person has to decide at what level they are comfortable. I like to see a good reward in exchange for a risk. The reward could be as simple as "it makes me happy", but that's not enough reward for much of a risk in my book.